how to solve this error

[Whiteneo]

when this kind of problem happens first at all, you have to verify your files by admin CP, to check and compare what files are changed, if you won't made any change you have to upload again wrong or damage files on this report.

Then you have to see you forum working again.

If you see all files are ok, that plugin has a problem and as Shade says you have to report this to his autor, sending all posible data they need to know. For example: MyBB version, theme with this problem, because all test i made bassed on default and clean installation or some theme and installation i know have no problems xD.